then both he and the buyer own the rights to that image
Not quite...the copyright only resides with one entity. The buyer may have unlimited access to reproduce the image (or whatever is specified by the copyright holder under the terms of agreement), but the copyright either resides with the original owner or is transferred completely to the buyer. Even with unrestricted access, unless the copyright is transferred to the buyer, the buyer typically does not have the ability to grant access to a third party to reproduce the image. It is the copyright owner who can designate how (and by whom) the image is used.